An often-heard complaint about GtkOptionMenu is that
the icons which appear in the image menu items in its menu are not
displayed in the button showing the selected item. This limitation
has been removed in GtkComboBox; the selected item appears in the
same way as the options in the popup.

Full tree model power

Since the new widgets are built around the same models that are
used for GtkTreeView, all of the powerful machinery of tree models
and cell renderers can be used.